Plasma multipactor

ABSTRACT

A multipactor tube includes a permeable anode positioned between dynode surfaces having a secondary emission ratio greater than one. An alternating voltage between the anode and dynodes causes ionization of a gas therein and results in continuous ion and periodic electron bombardment of the secondary emissive surface. A supply of electrons is maintained in the space to permit collection by the anode during intervals between electron bombardment. This facilitates self starting and sustaining of the multipactor oscillation and multiplication operation.

States Patent 1 [56] References Cited UNITED STATES PATENTS 2,163,7566/1939 Llewellyn 331/92 2,381,012 8/1945 Stutsman 331/92X PrimaryExaminer-Roy Lake Assistant Examiner-Siegfried H. Grimm Attorneys-C.Cornell Remsen, Jr., Walter .1. Baum, Percy P.

Lantzy, Philip M. Bolton, Isidore Togut, Charles L. Johnson, Jr. andHood, Gust, lrish & Lundy ABSTRACT: A multipactor tube includes apermeable anode positioned between dynode surfaces having a secondaryemission ratio greater than one. An alternating voltage between theanode and dynodes causes ionization of a gas therein and results incontinuous ion and periodic electron bombardment of the secondaryemissive surface. A supply of electrons is maintained in the space topermit collection by the anode during intervals between electronbombardment. This facilitates self starting and sustaining of themultipactor oscillation and multiplication operation.

PATENTEBunv 1s ISYI 6 21 .454

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ATTORNEYS.

PLASMA MULTIPACTOR BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates generally to multipactors and more particularlyto multipactors containing gas at a relatively high pressure whichinsures the stable and reliable operation thereof.

2. Description of the Prior Art Hard vacuum multipactors areconventional and well known and ordinarily include as essential featuresopposed dynode surfaces having a permeable anode structure therebetween.A cloud of electrons is oscillated by potentials applied to theelectrodes and are caused to strike repeatedly the dynode surfaces withsufficient velocity to release secondary electrons at a ratio greaterthan unity therefrom. A current flow from a DC source of potentialincluded in a circuit with the electrodes is produced. Included withinthis circuit is a tuned circuit which is excited by the current flow toproduce an alternating potential on the electrodes. During one portionof this alternating potential, electron multiplication occurs and duringanother, electrons are collected by the anode and are substantiallyswept out of the interelectrode space. As the next favorable portion ofthe alternating potential occurs, electron multiplication repeats andonce again is followed by an electron collection or quenching. Theenergy for maintaining this cycle of operation is, of course, derivedfrom the DC source which must be sufficiently high in potential topermit energy exchange processes resulting in the release of secondaryelectrons at the required ratio.

In the past, considerable difficulty has been experienced in renderingthis oscillatory action self-starting and self-sustaining, thisdifficulty arising from the fact that usually the efficiency of electroncollection is so high that few electrons are available to start electronmultiplication at the times of the favorable portions of the alternatingpotential. This invention is directed to overcoming this difiiculty.

SUMMARY OF THE INVENTION In accordance with the broader aspects of thisinvention, there is provided in a multipacting apparatus opposed dynodesurfaces having an anode electrode therebetween. A sealed envelope forthese electrodes is provided with a quantity of ionizable gas at arelatively high pressure in the range of from 1O to about Torr. Firstmeans are provided for energizing the dynode surfaces and the anode tocause continuous ion and periodic electron bombardment of the dynodesurfaces at energies which produce secondary emission therefrom. Thisenergizing means includes other means for causing the anode to collectthe electrons produced by and in the intervals between the periodicelectron bombardment. However, during this period of collection andsubstantially thereafter, the energizing means causes the ionbombardment to continue and to release a sufficient number of electronswhich are useful in initiating growth of an electron population.

It is, therefore, an object of this invention to provide a multipactorin which electron population growth is initiated by bombarding dynodesurfaces with ions.

It is another object of this invention to provide a multipactor in whichan ionizable gas at elevated pressure is utilized for the purpose offacilitating the electron multiplication process.

Still further, it is an object to facilitate self excited oscillationsin a multipactor by providing a predetermined quantity of gas therein.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawings, wherein:

FIG. I is a longitudinal sectional view of one embodiment of thisinvention;

FIG. 2 is an end section taken substantially along Section line 2-2 ofFIG. 1;

FIG. 3 is a cross section view taken substantially along line 33 of FIG.ll; 1

FIG. 4 is a schematic diagram of a self-excited oscillator whichincludes the tube of the preceding figures; and

FIGS. 5, 6, 7 and 8 are graphs used in explaining the operation of theinvention.

Referring to the drawings, and more particularly to FIGS. l, 2 and 3, ahermetically sealed glass envelope III has mounted in the opposite endsthereof cylindrica'lly shaped metallic terminal blocks Ill and I2 whichmount rigidly a plurality of molybdenum or like rods 13 cylindricallyarranged in spaced relation. These rods 13 are conductively connectedtogether at the ends thereof by means of the terminal blocks II and I2and constitute the anode-electrode of the tube.

Surrounding the rods 13 in coaxial relationship is a metallic sleeve Mwhich serves as the dynode, this sleeve 14 being made of a materialhaving a secondary emission ratio greater than unity. The sleeve 14 isrigidly mounted in place by means of a pair of glass or the like rings15 and 16 secured to the sleeve and the envelope as shown.

A suitable ionizable gas is contained within the envelope 10 at aconsiderably elevated pressure (as compared with hard vacuummultipactors of the prior art) in the pressure range of about 10 toabout 10 Torr. These limits are neither precise nor critical but maydepart therefrom to an extent as is explained more fully later on.

The tube thus far described may be incorporated into a selfexcitedoscillator, a typical circuit being shown in FIG. 4. In this embodiment,a tank circuit 17 tuned to a frequency in the spectrum of 3 to 10 MHz.is series connected between the anode 13 and a source of supplypotential as shown. A radio frequency choke 18 is connected between thetank circuit 17 and the supply. Power output from the tank circuit 17may be obtained by means of an inductor l9 suitably coupled thereto. Acathode (dynode) 14 as well as the negative terminal of the supplypotential are grounded.

In initiating operation of the circuit, the residuum of chargedparticles normally present within the envelope 10 are accelerated towardthe electrodes 13 and 14 upon application of the supply potential to thecircuitry. [It should be noted that the magnitude of the supplypotential determines in part the transit time of the electronsdiametrically across the dynode I4. Residual electrons in the spacebetween anode I3 and dynode 14 are oscillated diametrically through thetube permeating the anode I3. Unless this circulatory electron currentbuilds to a magnitude that induces sufficient current in the anode 13circuit to excite the tuned circuit 17, the total circuit will not breakinto oscillation. However, if and when the circulatory electron currentdoes reach a sufficient value, current drawn by the anode I3 energizesthe tuned circuit 17 thereby resulting in the application of analternating potential between the anode and dynode. In the case of FIG.4 where the dynode is shown grounded, the potential of the anode isdescribed by the solid curve of FIG. 5 and consists of an alternatingcomponent superimposed upon the DC source potential. Sustainedoscillation can be realized also if the tuned circuit is inserted intothe dynode leg, i.e., between the dynode and ground, rather than in theanode leg. In this arrangement the anode remains at a potential nearsource value and the potential variation of the dynode is of the formindicated by the dashed curve of FIG. 5. For explanatory purposes, thesolid curve of FIG. 5 will be utilized which applies to the circuitarrangement as shown in FIG. 4. Electrons traversing the structurediametrically during the intervals 0-(rr/2) and (31r/2-21r experience acentral force field having a positive time derivative in the regionbetween the dynode and anode. Within the anode structure a nearly fieldfree condition exists such that the kinetic energy of the electron,during this portion of trajectory remains virtually unchanged. The netresult, under these conditions, is that the electron gives up energy tothe time varying field during transit. Thus an electron emanating froman element of the dynode surface is unable to reach the opposite dynodesurface, or, in fact, any point on the cylindrical dynode whichrepresents a surface: of constant potential energy.

Conversely, during the interval (1r/2)(31r/2) when the time derivativeof the central force field is negative in the dynode-anode region, theoverall action of the field during transit is to impart kinetic energyto the electron so that it may impact the dynode surface 14 with energysufiicient to release secondaries. By causing the transit time for onediameter across the dynode to be considerably shorter than one period ofthe frequency of the tuned circuit 17, the electrons traveldiametrically of the dynode several times releasing secondaries on eachoccasion and thereby causing a rapid growth of the electron population.Thus, at the instant corresponding to three halves wavelength of theapplied potential, the growth will have been a maximum. Immediatelyfollowing, the anode potential increases thereby terminating theelectron bombardment of the dynode and causing the collection of theelectrons by the anode. The current thus produced in the anode circuitshock excites the tuned circuit 17 enhancing the alternating potentialbuildup which can be coupled from the circuit 17 by means of theinductor The action thus far described is conventional and well knownand is attended by the difficulty that once the electrons have beenswept out of the space by the anode 13 during the electron-collectingportion of the alternating potential, there are few, if any, electronsavailable to start the electron population growth during the nextsucceeding electron-multiplying portion of the potential. Consequently,the self-excitation and self-sustaining of oscillations has, in priorart devices, proven to be a problem.

In this invention, the gas is ionized to a small extent at the instantthe power is applied to the circuit, the ions being accelerated towardthe dynode l4 impacting the same with sufficient velocity to producesecondary electron emission. In this invention, the gas is ionized to asmall extend by natural radioactivity and cosmic radiation at theinstant power is applied to the circuit. The application of powerincreases slightly the number of electrons resent through gasamplification and ion bombardment of the dynode. If the tuned circuit 17is momentarily excited by external means, electron population growth isenforced which in turn generates a copious supply of ions viaelectron-atom collisions. Once this ion generation process has beeninitiated, the system is self-oscillating since the ions continue tobombard the dynode 14 releasing secondaries beyond the period ofelectron quenching. Thus, at the start of the favorable portion of thealternating potential, namely that portion corresponding to aquarter-wavelength, there is an adequate supply of electrons availableto start immediately the electron-multiplying action which continuesthroughout that portion of the cycle until the three-quarters wavelengthpoint is reached. Thus, instead of the tube being starved for electronsto initiate the population growth during the favorable portion of theapplied voltage, an ample supply of electrons is available. Thiselectron supply results from the presence of gas within the tube andtheoretically is due to ion bombardment of the dynode.

It has been found experimentally that the tube, in operation, possessesa strong dependence on gas pressure as indicated in FIG. 6 and that itis extremely difficult to start the multipactor at a pressure below 10'Torr. In the same model, it has been found that the approximate transittimes of ions from the anode region to the dynode have been determinedto be two to three times the period of an RF cycle. From this, it istheorized that energetic ions impacting the dynode surface give rise tosecondary emission which serves to initiate multipacting during thefavorable portion of each successive RF cycle. Further, it has'beenfound that multipactors having an anode of 2 cm. radius and a dynode of3.5 cm. radius are capable of operating over'a range of about 3 to 10MHz. If all of the operating parameters are held constant except foroperating frequency, at very low frequencies! there will be manyelectron transists during the multipacting interval (between the pointsof onequarter wavelength and three-quarters wavelength) but the energygained per transit will be small. As frequency is diminished, a pointwill ultimately be reached at which secondary electron yield issufficiently low that system losses cannot be overcome and operationceases. Similarly, at high frequencies, the magnitude of the chargecloud generated also diminishes, resulting eventually in cessation ofoscillation. In this case, energy gained per transit increases withfrequency, but the number of transits becomes reduced to a point wherean adequate charge cloud cannot be generated.

The effect of operating frequency on the magnitude of charge cloudgeneration, or overall gain of a multipacting process, is reflected inthe average anode current of the multipactor. FIG. 7 illustrates therelationship observed experi mentally, where parameters other thanoperating frequency were held constant.

In the forgoing as well as the appended claims, the gas pressure forobtaining the stable operating conditions described have been stated torange from about 10 to I0" Torr. These figures, as previously stated,are not critical but are close approximations. To further define theselimits, at some point between the pressure of 10" to 10" Torr, thestable operating conditions are lost. At the higher pressure levels, andat a value somewhere between 10 and 10" Torr, too much gas is presentwithin the tube such that a glow discharge results which interferes withthe stable operating conditions. Thus, by the delineation of 10 to 10"Torr as the range of useful pressures, it is intended that valuesslightly greater and slightly less than these limits be included. Theyare also included within the scope of the claimed invention.

In FIG. 8 is illustrated an approximation of the recurrence ofcirculatory current increases in the form of sharp pulses which, due tothe presence of the gas within the tube, are selfinitiating andrepetitive.

This invention need not be limited to use as a self-excited oscillatorbut will function equally well in common amplifier circuits.

While there have been described above the principles of this inventionin connection with specific apparatus, it is to be clearly understoodthat this description is made only by way of example and not as alimitation to the scope of the invention.

What is claimed is:

1. Electron discharge apparatus comprising opposed dynode surfaces of amaterial having a secondary emission ratio greater than unity, an anodebetween said surfaces permeable to the flow of charged particles, asealed envelope containing said dynode surfaces and said anode, anionizable gas at a pressure in the range of from about l0 to about 10Torr in said envelope, first means for energizing said dynode surfacesand said anode to promote ionization of said gas and to cause continuousion and periodic electron bombardment of said dynode surfaces atenergies which produce secondary emission therefrom, said first meansincluding second means for causing said anode to collect the electronsproduced by and in the intervals between said electron bombardment.

2. The electron discharge apparatus of claim 1 in which said first meansincludes a source of alternating voltage applied between said anode andsaid surfaces, said alternating voltage having a period longer than thetransit time of an electron between said surfaces.

3. The apparatus of claim 2 in which said source includes a resonantcircuit coupled between said surfaces and said anode and said secondmeans includes a source of unidirectional voltage.

4. The apparatus of claim 3 in which said dynode surfaces are part of acylindrical dynode and said anode includes a series of cylindricallyarranged and spaced-apart rods concentrically disposed within saiddynode, said resonant circuit including a tank circuit series connectedbetween said source of unidirectional voltage and one of said anode anddynode.

5. The apparatus of claim 3 in which said resonant circuit is tuned to afrequency in the range from about 3 MHz. to about 10 MHz. at which anodecurrent is maximized.

6, The method of producing electrical signals comprising the steps offorming clouds of ions and electrons between spaced-apart dynodesurfaces, applying an electric field to the space between said surfaceswhich impacts said ions with said surfaces but oscillates said electronstherebetween, said field imparting energies to said ions which releasesecondary electrons 'from said surfaces that join said electron cloud,applying a varying accelerating field to said electrons which bombardsthem periodically against said surfaces with secondary emittingenergies, collecting at least a portion of the electron cloud during thetime the electrons are not bombarded against said surfaces, deriving asignal from the act of electron collection, and continuing the impactingof said surfaces by said ions during the period between electronbombardments thereby maintaining a supply of electrons in the spacebetween said dynodes.

7. The method of claim 6 wherein said ions are formed of gas at apressure of from about 10" to about 10" Torr.

8. The method of claim 6 wherein said electric field is unidirectionaland applied in such polarity as to provide a potential gradient whichincreases from a minimum at said dynode surfaces toward a positivemaximum at a location therebetween, said varying field having a periodin excess of the time of transit of an electron in one trip from onedynode surface to the other.

1. Electron discharge apparatus comprising opposed dynode surfaces of amaterial having a secondary emission ratio greater than unity, an anodebetween said surfaces permeable to the flow of charged particles, asealed envelope containing said dynode surfaces and said anode, anionizable gas at a pressure in the range of from about 10 3 to about 105 Torr in said envelope, first means for energizing said dynode surfacesand said anode to promote ionization of said gas and to cause continuousion and periodic electron bombardment of said dynode surfaces atenergies which produce secondary emission therefrom, said first meansincluding second means for causing said anode to collect the electronsproduced by and in the intervals between said electron bombardment. 2.The electron discharge apparatus of claim 1 in which said first meansincludes a source of alternating voltage applied between said anode andsaid surfaces, said alternating voltage having a period longer than thetransit time of an electron between said surfaces.
 3. The apparatus ofclaim 2 in which said source includes a resonant circuit coupled betweensaid surfaces and said anode and said second means includes a source ofunidirectional voltage.
 4. The apparatus of claim 3 in which said dynodesurfaces are part of a cylindrical dynode and said anode includes aseries of cylindrically arranged and spaced-apart rods concentricallydisposed within said dynode, said resonant circuit including a tankcircuit series connected between said source of unidirectional voltageand one of said anode and dynode.
 5. The apparatus of claim 3 in whichsaid resonant circuit is tuned to a frequency in the range from about 3MHz. to about 10 MHz. at which anode current is maximized.
 6. The methodof producing electrical signals comprising the steps of forming cloudsof ions and electrons between spaced-apart dynode surfaces, applying anelectric field to the space between said surfaces which impacts saidions with said surfaces but oscillates said electrons therebetween, saidfield imparting energies to said ions which release secondary electronsfrom said surfaces that join said electron cloud, applying a varyingaccelerating field to said electrons which bombards them periodicallyagainst said surfaces with secondary emitting energies, collecting atleast a portion of the electron cloud during the time the electrons arenot bombarded against said surfaces, deriving a signal from the act ofelectron collection, and continuing the impacting of said surfaces bysaid ions during the period between electron bombardments therebymaintaining a supply of electrons in the space between said dynodes. 7.The method of claim 6 wherein said ions are formed of gas at a pressureof from about 10 3 to about 10 5 Torr.
 8. The method of claim 6 whereinsaid electric field is unidirectional and applied in such polarity as toprovide a potential gradient which increases from a minimum at saiddynode surfaces toward a positive maximum at a location therebetween,said varying field having a period in excess of the time of transit ofan electron in one trip from one dynode surface to the other.